Synapses in the central nervous system undergo various short- and long-term changes in their strength, but it is often difficult to distinguish whether presynaptic or postsynaptic mechanisms are responsible for these changes. Using patch-clamp recording from giant synapses in the mouse auditory brainstem, we show here that short-term synaptic depression can be largely attributed to rapid depletion of a readily releasable pool of vesicles. Replenishment of this pool is highly dependent on the recent history of synaptic activity. High-frequency stimulation of presynaptic terminals significantly enhances the rate of replenishment. Broadening the presynaptic action potential with the potassium-channel blocker tetraethylammonium, which increases Ca2+ entry, further enhances the rate of replenishment. As this increase can be suppressed by the Ca2+-channel blocker Cd2+ or by the Ca2+ buffer EGTA, we conclude that Ca2+ influx through voltage-gated Ca2+ channels is the key signal that dynamically regulates the refilling of the releasable pool of synaptic vesicles in response to different patterns of inputs.

Using a combination of patch‐clamp, in situ hybridization and computer simulation techniques, we have analysed the contribution of potassium channels to the ability of a subset of mouse auditory neurones to fire at high frequencies. Voltage‐clamp recordings from the principal neurones of the medial nucleus of the trapezoid body (MNTB) revealed a low‐threshold dendrotoxin (DTX)‐sensitive current (ILT) and a high‐threshold DTX‐insensitive current (IHT). I HT displayed rapid activation and deactivation kinetics, and was selectively blocked by a low concentration of tetraethylammonium (TEA; 1 mm). The physiological and pharmacological properties of IHT very closely matched those of the Shaw family potassium channel Kv3.1 stably expressed in a CHO cell line. An mRNA probe corresponding to the C‐terminus of the Kv3.1 channel strongly labelled MNTB neurones, suggesting that this channel is expressed in these neurones. TEA did not alter the ability of MNTB neurones to follow stimulation up to 200 Hz, but specifically reduced their ability to follow higher frequency impulses. A computer simulation, using a model cell in which an outward current with the kinetics and voltage dependence of the Kv3.1 channel was incorporated, also confirmed that the Kv3.1‐ like current is essential for cells to respond to a sustained train of high‐frequency stimuli. We conclude that in mouse MNTB neurones the Kv3.1 channel contributes to the ability of these cells to lock their firing to high‐frequency inputs.